Incorrect has-type on unpacking of known TypeVarTuple type from classmethod from class with contrained generic
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Description
Bug Report
When trying to unpack a tuple of a TypeVarTuple from a class method where that class contains a constrained TypeVar different from the one inhabited by the TypeVarTuple, mypy will issue a has-type error despite reveal_type showing the full correct type.
To Reproduce
https://mypy-play.net/?mypy=master&python=3.14&gist=856510a8671b57985553a6439ee673dc
class Container[T, P: (Literal[True], Literal[False])]:
def foo(self) -> T:
raise NotImplementedError
def f[*Ts, P: (Literal[True], Literal[False])](
container: Container[tuple[*Ts], P],
) -> None:
result = container.foo()
reveal_type(result) # Revealed type is "tuple[*Ts]"
y = (*result,) # Cannot determine type of "result" [has-type]
# Expected: tuple[*Ts]
# Actual: tuple[Any, ...]
reveal_type(y)
Expected Behavior
This code should have no errors, and the type of y should be tuple[*Ts]
Actual Behavior
Despite the reveal_type of result clearly showing tuple[*Ts], mypy issues a has-type error when unpacking it, and the resulting type of y ends up being wrong.
Additional Notes
If you remove the P generic, or make it normal/bound instead of constrained, the code works as expected.
Adding an annotation to result does get rid of the error, however in my actual code the result from foo is a tuple that I unpack like this result, index = container.foo(), so I cannot easily add that annotation. (I can like this result: tuple[*Ts] before the unpacking assignment, but that makes the code uglier.)
Your Environment
- Mypy version used: Playground 2.1.0 / master
- Mypy command-line flags: None
- Mypy configuration options from
mypy.ini(and other config files): None - Python version used: 3.14
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the linked mypy-play reproducer and compare the constrained TypeVar case with the working variants that remove or change P. Trace how the revealed tuple[*Ts] is handled during starred unpacking. Done means the reproducer produces no has-type error and y is inferred as tuple[*Ts], with a regression test covering the case.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- compilers, devtools
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Quiet
- Clarity
- Mostly clear
- Newbie friendliness
- 48/100